Amplifying system



July 25, 1944. P. c. GARDINER 2,354,514

AMPLIFYING SYSTEM Filed Jan. 23, 1942 Inventor: Paul C.Gar-diner-, b WW5) His Attorney.

Patented July 25, 1944 mum-me SYSTEM Paul C. Gardiner, Scotia, N. Y., assignor to General Electric Company, a corporation of New York Application January 23, 1942, SeriaLNo. 427,972

6 Claims.

My invention relates to amplifying systems and has for one 01 its objects the provision of an r improved, simplified arrangement for neutralizing regenerative feedback through the interelectrode capacity of a high frequency signal amplifier.

Another object of my invention is to provide a tuned high frequency amplifier stage, particularly of the intermediate frequencytype, in which the gain, stability, and jrequency response characteristics are improved through the use of a novel neutralizing network interconnecting its input and output circuits.

It is also an object of my invention to provide improved means for neutralizing each stage of a tuned high frequency amplifying system independently, so that circuit alignment and operating adjustments of the system are facilitated.

Still another object of my invention is to provide an improved high frequency amplifier where in existing circuit elements in its output circuit are utilized to supply feedback voltages to its in-' put circuit in proper phase relationship to effect neutralization, thereby eliminating the need for special feedback coils, or the like, and reducing the cost of the amplifier.

The features of my invention which I believe to be novel are set forth with particularity in the appended claims. My invention itself, however, together with further objects and advantages thereof, may best be understood by reference to the following description taken in connection with the accompanying drawing, in which the single figure thereof is a circuit diagram of one embodiment of the invention.

The amplifier In and its associated circuits com-.

prise one stage of a high frequency signal amplifying system such as may be included, for example, in the intermediate frequency amplifying circuit of a superheterodyne radio receiver.

The amplifier I is represented as a pentode having a cathode ll, control grid l2, screen grid I3, suppressor grid [4, and anode l5. The anode circuit for the amplifier Ill includes the first winding of a three-winding intermediate frequency transformer It. The design and construction of an interstage coupling transformer ofv this type, having three or more windings, will be apparent to those skilled in the art without detailed explanation. The frequency response characteristics of such transformers are particularly suited to the requirements of amplifiers operating at the higher radio frequencies.

Briefly, the transformer l6 comprises three inductive windings II, III and I9 each of'which is tuned to the operating frequency of the system by the capacitors 20, 2| and 22, respectively. The three windings are reactively coupled together in cascade relationship. In other words, the winding II, which is included in the anode circuit of the amplifier II, is inductively coupled to the winding i8, and the winding I8 is in turn inductively coupled to the winding l9. It is also generally preferable to enclose the transformer it in a grounded electrostatic shield, usually in the form of a metal container enclosing the transformer assembly. Such an electrostatic shielding means is indicated schematically in the drawing by the dashed rectangle 23. The container 23 may also be'sectionalized to provide electrostatic shielding partitions between adjacent transformer windings as indicated schematically by the dashed lines 24, 25.

The input circuit for the amplifier l0 includes an inductive winding 30 tuned to the operating frequency of the system by the shunt capacitor 3i. High frequency waves to be amplified are supplied from any suitable source, not shown. For example, the tuned circuit 30, 3| may comprise the last section of a transformer similar to the transformer I5, and the signals may be supplied from a prior intermediate frequency amplifier stage or from a frequency converter.

The amplifier III is coupled to a suitable load which, in the illustrated embodiment, comprises another identical stage of intermediate frequency amplification. Thus the control grid of the pentode amplifier 32 is connected to the third tuned circuit I9, 22 of the transformer l5, and the anode of the amplifier 32 is connected to a tuned circuit 33, 34 which may comprise the first section of a coupling transformer similar to the transformer 15.

A suitable source of anode and screen grid potentials for the amplifiers l0 and 32 is conventionally represented by a battery 40 shunted by a filter capacitor 4| and a voltage dividing re- .sistor 42. As is customary the screen grid and anode circuits are provided with the usual decoupling resistors 43-45 and bypass capacitors 41-50. The amplifiers l0 and 32 are respectively self-biased by the resistors 5| and 52 connected between their cathodes and ground and bypassed by the capacitors 53 and 54. The control grid circuits for the amplifiers are illustrated as being returned to ground through a suitable source of automatic volume control potentials, not shown The usual filter resistors 55 and 55 and bypass capacitors 51 and 58 are also included in these circuits. The purpose and arrangement of these various circuit elements and power supply connections will be readily apparent to those skilled in the art, and they have been mentioned only briefly for the sake of completeness.

It is well known that one of the factors which limits the maximum amplification obtainable in a single amplifier stage is the regenerative action inherently. present in the amplifier as a result of feedback through the interelectrode tube capaci- V ties, particularly the anode-to-control grid'capacity of the amplifying device employed. This inherent interelectrode capacity has been indicated schematically in the drawing in dashed outline by the capacities 60 and GI between the anode and control grids of the respective amplifiers. This interelectrode capacity serves to couple the output circuit of each device to its input circuit and eventually leads to parasitic oscillations in the amplifier stage as the gain is increased beyond a certain point. Even in the absence of oscillations the detrimental effect of this regenerative feedback upon the frequency response characteristics of the system is familiar to the art.

In accordance with my invention the regenerative voltages impressed upon the control grid circuit of the amplifier ll through the interelectrode tube capacity 60 are compensated or neutralized by degenerative voltages fed back externally from a selected point in the output transformer it. In a practical amplifier construction the proper phase and magnitude of the neutralizing voltages are readily secured by the use of a single additional conductor 62 having one end connected to the control grid 12 and its other end extending through the electrostatic shield 23 into capacitive relationship with the left-hand end of the winding It, as is illustrated in the drawing. The capacity between the conductor 62 and the winding I8 is indicated schematically in the drawing by the dashed lines at 63.

It is well known that the external feedback voltage must differ in phase by substantially 180 electrical degrees from the anode voltage of the amplifier for correct neutralization. That this phase relationship is secured in the illustrated embodiment will readily become apparent from a consideration of the phase shifts through the successive windings of the transformer l6. Since the transformer windings are coupled together in cascade relationship by the mutual inductance between windings I I and i8 and between windings ll and i8, respectively, the high frequency signal voltages across the winding ll are substantially 90 degrees out of phase with the high frequency voltages across the winding [8. Similarly, the phase of the high frequency voltages across the winding i9 is further displaced in the same direction by substantially 90 degrees from the phase of the voltages across the winding l8, making the total phase shift through the transformer substantially equal to 180 degrees.

end of conductor 2 into capacitive relationship with the winding I 8, as previously described. The value of this capacity is also very readily adjusted by varying the position of the conductor 82 within the shield 23.

Obviously, the other amplifying stages in a multi-stage amplifier embodying the invention may be individually neutralized in the same manner. A neutralizing connection 84 is indicated in the drawing for supplying neutralizing voltages to the control grid of the amplifier 32 from a subsequent winding on its output transformer, not illustrated.

It will now be seen that I have provided a very simple and effective neutralizing arrangement for a high frequency amplifier which requires a minimum of circuit elements in addition to those already utilized for other functions. The need for special neutralizing windings, or the like, is obviated. Furthermore, each amplifying stage can be aligned and neutralized independently of any other stages in the system,which greatly simplifies the alignment procedure.

While reference has been made to the specific application of my invention to the intermediate frequency circuits of a superheterodyne receiver, it will of course be understood that it is not limited thereto but is generally applicable to tuned amplifiers of this general type having one or more stages utilizing coupling transformers provided with at least three cascaded windings. Therefore while I have shown a particular embodiment of my invention, I contemplate by the appended claims to cover any modifications thereof which fall within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the United States, is:

1. In a system for amplifying waves of a predetermined frequency, the combination of an electron discharge device having input and output circuits between which undesired regenerative feedback exists by reason of in'terelectrode capacity in said device, at least three resonant circuits each tuned to the operating frequency of the system, the first one of said resonant circuits being included in said output circuit, said resonant circuits being coupled together in cascade arrangement for the successive transfer of high frequency waves between said output circuit and a load device, and a capacity coupling between said input electrode and a point on the third one of said resonant circuits in said cascade arrangement for substantially neutralizing said regenerative feedback.

2. A high frequency amplifier comprising, in

- combination, a thermionic amplifying device having control grid and anode circuits, said grid voltage impressed thereon from said anode circult through interelectrode capacity in said de- In order for the voltage impressed uponthe I Therefore it is readily provided by extending the 7s vice, an output transformer having at least first, second and third windings each tuned to the same operating frequency, said first winding being connected in said anode circuit and inductively coupled to said second winding, said second winding being inductively coupled to said third winding, whereby a progressive phase shift is produced in the high frequency voltages across successive windings, and capacitive means for degeneratively impressing a voltage upon said control grid which is supplied from said third winding and which is sufiiclent to substantially neu-- tralize said regenerative feedback voltage.

3. A high frequency amplifier comprising, in

amplifier having a control grid and an anode between which undesired capacity exists, first 7 combination, a thermionic amplifying device having input and output circuits, said input circuit having regenerative feedback voltage impressed thereon from said output circuit through interelectrode capacity in said device, an output transformer having at least three windings each tuned to the same operating frequency, said first winding being included in said output circuit, said windings; being reactively coupled together in cascade arrangement for the successive transfer of high frequency energy between said output circuit and a load device, whereby a progressive phase shift is produced in the high frequency voltages across successive windings, and capacitive means for impressing upon said input circuit a voltage which -is supplied from one of said windings and which is in degenerative phase with respect to said feedback voltage for substantially neutralizing said feedback voltage.

4. A neutralized high frequency amplifying stage, comprising, in combination, a thermionic amplifier having input and output electrodes between which undesired capacity exists, first and second resonant circuits tuned to the same high frequency and connected respectively to said electrodes, means for supplying waves of said high frequency to be amplified to said first circuit,

a third resonant circuit tuned to said frequency and reactively coupled to said second circuit in such sense that the high frequency voltages thereacross are substantially 90 degrees out orphase with the high frequency voltages across said second circuit, a fourth resonant circuitv timed to said frequency and reactively coupled to said third'circuit in such sense that the high frequency voltages thereacross are substantially 180 degrees out of phase with the high frequency voltages across said second circuit, and means for impressing said fourth circuit voltages across said firstcircuit in sufilcient intensity to neutralize voltages impressed thereon through said capacity.

'5. A neutralized high frequency amplifying stage comprising, in combination, a thermionic and second resonant circuits tuned to the same high frequency and connected respectively to said grid and anode, means for supplying waves of said frequency to be amplified to said first circuit, a third resonant circuit tuned to said frequency and inductively coupled to said second circuit, a fourth resonant circuit tuned to said frequency and inductively coupled to said third circuit, whereby the high frequency voltages developed across said fourth circuit are substantially 180 degrees out of phase with the high frequency voltages developed across said second circuit, and means for impressing said fourth circuit voltages across said first circuit to neutralize voltages impressed thereon through said capacity, said last means comprising a capacity connected from a point on said fourth circuit to said control grid.

6. A high frequency amplifier comprising, in combination, a thermionic amplifying device having input and output circuits, said input circuit having regenerative feedback voltage impressed thereon from said output 'circuit through interelectrode capacity in said device,. an output transformer having first, second and third windings each tuned to the same operating frequency, said first winding being included in said output circuit, said windings beingreactively coupled together in cascade arrangement for the successive transfer of high frequency energy between said output circuit and a load device whereby a progressive phaseshift is produced in the high frequency voltages across successive windings, and means including a conductor connected at one end to said input circuit and having its other end arranged in capacitive relationship with said third winding for impressing upon said input circuit a voltage which is in degenerative-phase with respect to said feedback voltage for substantially neutralizing said feedback volt- PAUL C. GARDINER. 

